The detection of phase modulation or frequency modulation of an optical wave is important from various fields of application where optical beams are used to detect the motion of objects. This is the case of laser sensing of vibrations and laser detection of ultrasound and of transient body deformations such as those produced by a shock or impact. Of particular interest for practical applications is the case where ultrasound or a shock wave is generated by a laser. In this case, a completely remote ultrasonic inspection system can be realized, permitting for example ultrasonic probing at elevated temperatures. A technique based on laser generation and optical detection can thus be advantageously used to inspect materials at high temperatures (such as all metals and ceramics) for process and quality control, to detect flaws as soon as they are created during processing, to measure production parameters such as thickness, temperature, etc. and to determine micro structural properties on-line (grain size, porosity, etc.) This technique is also particularly advantageous to inspect parts of complex shapes such as those made of polymer matrix composite materials and used in advanced aeronautic and aerospace structures.
In all cases of practical interest, ultrasonic excitation of the object produces at its surface, very small displacements that translate into correspondingly very small phase or frequency perturbations. Therefore, a sensitive detection technique has to be used, which means in practice a technique based on optical interferometry. Since in practice, the probed surfaces are rough, the ultrasonic information is encoded into an optical beam with speckle and a suitable interferometric technique should integrate effectively over the whole speckle field or provide demodulation independently of speckle nature of the collected light beam. In various U.S. patents, Applicant has described interferometric schemes for sensitive detection in these conditions. All of these schemes are characterized by their large etendue parameter.
This etendue parameter (or throughput), is defined as the product of its effective entrance aperture area by the solid angle limited by the rays of maximum inclination passing through the entrance aperture center and thus defining the field of view. The maximum inclination rays can be defined as those which produce a shift of the interference pattern by a quarter of a fringe. The importance of the etendue parameter stems from its invariance within the frame of geometric optics. A large etendue permits to choose light collecting optics of large size, being only limited by cost and practical feasibility, and to detect surface motion over a large area.
In an arrangement described by the applicant in U.S. Pat. No. 4,659,224 issued Apr. 21, 1987, entitled Optical Interferometric Reception of Ultrasonic Energy, a confocal Fabry-Perot is used in transmission to provide a signal representative of the surface motion independently of the speckle effect. In U.S. Pat. No. 4,966,459 issued Oct. 30, 1990, entitled Broadband Optical Detection of Transient Surface Motion From a Scattering Surface, the applicant describes the use of the same type of interferometer, that may be used within a Mach-Zehnder interferometric arrangement or in a reflection scheme to provide the same capability with a very broad detection bandwidth. Still a broader detection bandwidth, especially including the low ultrasonic frequency range, several KHz to about 1 MHz, is described by the applicant and R. K. Ing in U.S. Pat. No. 5,131,748 issued Jul. 21, 1992, entitled Broadband Optical Detection of Transient Motion from a Scattering Surface that is based on the use of two-wave mixing in a photorefractive crystal.
Although the prior art described performs its intended function adequately, it only provides information relating to a single location on the surface of the object. In order to obtain an ultrasonic image of the object representative of a property of interest, such as thickness, porosity, microstructure, presence and location of flaws, the detection laser beam is usually scanned with a rotating mirror over the surface of the object. Alternatively the object can be moved in front of the laser beam. Although such an approach works properly, it is slow and can take precious time if a large surface area is to be scanned. The present application describes a method and apparatus that can generate an ultrasonic image of the object without using optical scanning and can simultaneously detect signals originating from multiple spots on the surface of an object.